During the last 15 years a new flow meter technology has evolved using vibrating conduits and Coriolis forces developed in fluids flowing therein to measure the mass flow rate of a fluid. A typical sensor employs two parallel conduits usually bent into some unique shape such as a "U" or an "S" or a bowtie shape. These conduits are normally welded to a rigid casting at their inlets and outlets and the conduits are forced to vibrate with one conduit vibrating in opposition to the other for balance purposes. Fluid flowing through the vibrating conduits therefore experiences this vibrating motion and, in particular, the angular rotation of the conduits, causing Coriolis forces to impinge on the walls of these conduits. The net effect is a slight deformation and deflection of the conduit proportional to the mass flow rate of the fluid, the angular velocity of the conduits and some conduit geometry constants. This deformation exemplifies itself and is normally measured as a small phase or time delay between the deflection at the inlet ends of the conduits compared to the deflection at the outlet ends.
Since its inception and reduction to practice in industry, many innovative methods and inventions have been employed to maximize these tiny deflections and their time delays to improve the sensitivity of these devices. Many of these efforts have focused on improving the shape of the bent conduits to maximize angular rotational velocity while minimizing the stiffness that opposes the Coriolis deflections. This line of reasoning has led to many innovative shapes that lengthen these phase delays, however the design tradeoffs are generally, (a) a complex conduit shape to bend, (b) thinner walled flow conduits, (c) a lower frequency of operation, creating susceptibility to ambient vibrations and (d) a higher pressure drop through the device due to introduction of flow splitters and complex conduit geometry. Some examples of these flow conduit geometries are the dual S-conduits of U.S. Pat. Nos. 4,798,091 and 4,776,220 to Lew, the .OMEGA.-shaped conduits of U.S. Pat. No. 4,852,410 to Corwon, et al., the B-shapes conduits of U.S. Pat. No. 4,891,991 to Mattar, et al., the helically-wound flow conduits of U.S. Pat. No. 4,756,198 to Levien, figure-8 shaped flow conduits of U.S. Pat. No. 4,716,771 to Kane and the dual straight conduits of U.S. Pat. No. 4,680,974 to Simonsen, et al.. These complex conduit shapes also contribute to the overall size of the device and usually preclude the ability to place the device within a reasonably-sized pressure-containing case for safety reasons. In addition to the problems associated with the general shape of the conduits, the magnitude and placement of masses on the conduits (such as magnets), for the purposes of driving and sensing the requisite vibratory motions, can adversely affect the response of the device to flow rate under the conditions of changing pressure, density or both.
Currently, several new types of Coriolis flow meters are being developed using radial vibratory motion of the wall of a single straight conduit, with flow going either within or around the conduit, thereby eliminating the flow splitters and the second counterbalancing flow conduit. Such meters are referred to as "radial mode meters."
A Coriolis mass flow meter sensor is a device that accommodates the flow of fluid within or around one or more flow conduits and subjects that flowing fluid to an angular rotation by virtue of a vibration or oscillatory motion of the flow conduit(s). The flow conduit(s) are normally designed to be highly resonant devices that can be easily excited to vibrate in one or more natural modes of vibration for this purpose. This angular rotation imparted into the flowing fluid by this conduit motion thus causes resultant Coriolis forces to develop and bear against the sidewalls of the flow conduit in a direction that is 90.degree. from both the angular rotation vector and the fluid velocity vector. These resulting forces are cyclic in nature (normally sinusoidal) at the frequency of the excitation motion.
Since these devices are normally designed to be highly resonant and compliant structures, the resulting sinusoidal Coriolis force distribution results in a sinusoidal (as a function of time) deflection of the structure that is proportional to the mass flow rate of the fluid and the frequency response of the structure for that given excitation. As stated previously, the frequency response of the device can change as a function of fluid pressure, density, temperature and conduit stress, it is essential that it be accurately known and compensated for, or held to be constant to achieve accurate mass flow rate measurements from the device. Therefore, one aspect of the present invention is the ability to control the response of a flow conduit geometry to achieve insensitivity to these changing parameters by virtue of the flow conduit's geometry in combination with a motion measurement method. This arrangement implemented on a radial mode meter has many advantages over the bending mode meter types previously described and virtually solves the design tradeoffs (a) through (d) previously mentioned. While both radial mode and bending mode meters can exhibit pressure sensitivity, the problem is generally more severe on radial mode Coriolis meters because internal fluid pressure tends to stiffen the walls of the conduit with regard to radial vibrations more than with regard to bending mode vibrations. In addition, radial mode meters generally have vibration frequencies that are orders of magnitude higher and have flow-related phase or time delays that are many times smaller than traditional bending mode Coriolis flow meters. For example, a typical 1" bending mode Coriolis flow meter operates at approximately 100 Hz while a typical 1" radial mode meter operates above 3000 Hz. Consequently, the Coriolis signal-processing method of choice traditionally has been a phase or time delay measurement. Unfortunately, measuring phase or time delay results in very poor sensitivity and resolution for radial mode meters. For example, a typical time delay measured at a nominal flow rate on a traditional bending mode flow meter is in the tens of microseconds and easily measured with today's electronics. By contrast, the same flow rate in a comparable radial mode Coriolis meter might cause only tens of nanoseconds of time delay, a period of time that is much more difficult and expensive to measure accurately. To date, the only radial mode Coriolis flow meter patent has been that of Lang (U.S. Pat. No. 4,949,583) who describes a radial mode vibrating sensor. However, Lang exclusively uses a relative phase displacement measuring technique requiring signal processing circuitry designed to discern minuscule time delays. Such circuitry is relatively expensive and inaccurate, damaging the efficacy of the meter as a whole. Consequently, to date no practical device has yet been disclosed or marketed for commercial applications.
Therefore, the current invention addresses the more general task of providing a Coriolis flow meter that is insensitive to both density and pressure at the same time.